Someone has done some pretty extensive testing (Chuyler?), and it might be stickied somewhere, comparing RH,RM and RS compounds and the RS compound won every time. The lap time advantage outweighs the tire wear disadvantage and extra pit stop(s) in longer races. Someone mentioned above that RS tires last just as long and that's not true if you are going at racing speed. It might be true if you run the same laptimes on both compounds but the advantage of RS tires is the lower lap times.
The key is making them last. Softer brake balances like 2/1 or 1/2 helps and barely affects lap times. I've won several longer races with brake balances like that. Don't oversteer the car, meaning don't turn the wheels beyond what is needed so make sure the car is tuned for a sharper front end so you aren't fighting understeer. Do a test run and find out what the car is like later in the run. You will probably find the front end wearing faster at Monaco so you might be able to adjust your tune around that. Adjust for oversteer at first and the car will come back to neutral later in the run.
You can run RS tires down pretty low before you need to change them but your target should be either 10 or 15 laps. If you can make it to 10 you need 2 stops. 15 is unlikely but you never know and that would be ideal. When you start losing about more than 3 or 4 seconds a lap that's the time to change the tires. Don't wait until they are bald and losing 5+ seconds a lap to save a pit stop because it won't pay in the end. And remember not to take fuel on in the pit stop. A full fuel load does affect laptimes.
Last but not least the golden rule of tire wear is:
Squealing tires are not happy tires!!
Tire squeal = tire wear in GT5 so drive as smooth as you can avoiding tire squeal and that will maximize tire life.
EDIT: Just noticed your picture. Take the wing off. It is completely unneccary at that PP level on racing tires and just creates understeer and robs you of HP.
EDIT2: Found that data:
https://www.gtplanet.net/forum/showthread.php?t=236549&highlight=tire+wear